A microfiltration filter generally has a membrane with a pore size or diameter between about 0.05 microns and about 10 microns (.mu.m) and which may be composed of synthetic resin materials.
Microfiltration membranes have been proposed in a variety of forms and can be constituted, according to the art, from various compositions. For example, mention may be made of cellulose esters, polysulfones, (European patent application No. 81 - 301 074/003 63 15) polyvinylchloride (PVC), aliphatic polyamides (U.S. Pat. No. 3,408,315 or the like.
While these materials have been found to be effective for many microfiltration processes and in membrane filters for use on a small scale and have limited applications, they generally have characteristics which have limited their wide-spread industrial use. For example, they may have limited chemical resistance, i.e. resistance to attack by corrosive substances, they may have poor resistance to moisture, and may have poor mechanical properties such as tearing strength and bending strength.
Strength problems in filters have been attacked by providing membranes which are composed of aromatic polyamides (German patent document DE-OS No. 26 42 979 and U.S. Pat. No. 4,229,291), although these membrane filters do not have a pore size in the range of about 0.05 microns to about 10 microns and are not suitable for use in microfiltration. Indeed, as far as we are aware, it has not been possible heretofore to provide such membrane filters suitable for microfiltration.